In the prior art, a structure equipped with right and left hydraulic transmission mechanisms (HST) for driving right and left traveling crawlers individually has the problem that straight-ahead traveling performance cannot be easily improved. Another structure equipped with a hydraulic transmission straight-ahead driving mechanism for driving the right and left traveling crawlers by operating a straight-ahead transmission lever and a hydraulic transmission swing mechanism for driving the right and left traveling crawlers by operating a steering wheel in order to transmit driving force of said straight-ahead driving mechanism and said swing mechanism to the right and left traveling crawlers through a forced-diff has the problem that, although the straight-ahead traveling performance can be easily improved by driving the right and left traveling crawlers with the straight-ahead driving mechanism, when both the hydraulic transmission swing mechanism and the hydraulic transmission straight-ahead driving mechanism are stored in a space under a machine floor, the machine floor becomes too high to provide space under the machine floor with an allowance for heat radiation. Another problem is that the forced-diff driven by the hydraulic transmission swing mechanism tends to become complicated, and installation of the swing mechanism with a heavy weight, or the forced-diff mechanism with a heavy weight, tends to result in an improper shift of the machine weight. Such additional problems of the crawler type traveling mechanism cannot be easily constructed using components of a wheel type traveling mechanism, the installation position of the swing mechanism is restricted, or a change in ground contact length of the traveling crawlers tends to result in a change in swing resistance.